Our long term objective is to elucidate the mechanisms involving modulation of spermatogenesis. To investigate the role of number of spermatogonia and amount of germ cell degeneration in modulation of spermatogenesis and the possible role of number of Sertoli cells on each, the number of each type of germ cell from type A spermatogonia to spermatids, the theoretical daily sperm production (DSP) based on the number of each type of germ cell and number of Sertoli cells will be determined throughout the annual reproductive cycle in stallions. The number of Leydig cells and circulating levels of FSH, LH and testosterone as well as ultrastructural characterization of Sertoli cells and Leydig cells will be evaluated in light of changes in spermatogonial abundance, germ cell degeneration and DSP. Seasonal modulation of spermatogenesis exhibited by stallions permits evaluation of testicular and hormonal changes which are involved in quantitative maintenance of spermatogenesis. We have shown that DSP increased by 67% and the number of Sertoli cells increased by 48% during the breeding season of 4 to 5 years old horses. Histometric analyses, which we have previously employed for quantification of spermatogenesis, will be used to determine germ cell production rates at different steps in spermatogensis and the magnitude of degeneration. Numbers of Sertoli and Leydig cells will be detemined in testicular homogenates or by histometric analyses. Sertoli cells and Leydig cells will be characterized by electron microscopy. Hormones will be evaluated with validated radioimmunoassay under subcontract by Dr. Thompson. Significance of this study stems from the fact that degeneration of germ cells during spermatogenesis is a comon phenomenon and reduces DSP to less than 50% of theoretical value; however, little is known about its mechanism, causes, relationship to the size of the Sertoli cell population or prevention. Knowledge of how sperm production may be increased has immediate application in animal breeding programs and in human family planning programs involving oligospermic men.